Ina Drew, far left, said she believed her oversight of the trades had been 'reasonable and diligent'. Photograph: Gary Cameron/Reuters

JP Morgan faced a barrage of criticism on Friday for it disastrous "London whale" trading loss as senators and the bank's regulator accused its executives of believing they were too big to fail, ignoring warnings about the escalating losses and deliberately withholding information.

At a hearing a day after it published a damning 300-page report on JP Morgan's $6.2bn debacle, the Senate subcommittee on investigations hammered bank executives over the affair, while regulators flatly denied JP Morgan's claims that it had kept them informed about the mounting losses.

The main witness at the day-long hearing, chaired by retiring Democratic senator Carl Levin, was Ina Drew, the former head of the chief investment office that oversaw the London trading operation. She has since resigned, while many of the other traders involved were fired. "Things went terribly wrong," she said, before deflecting the blame to the traders below her. She insisted that key facts "were not brought to my attention at the time".

Levin and Republican senator John McCain seemed more concerned with the bankers still at JP Morgan, specifically chief executive Jamie Dimon, who did not testify. The senators said that Dimon, who has previously avoided direct blame for the losses, had deliberately stopped regulators being given information on the bank's positions in August 2011.

"Firing a few traders and their bosses won't be enough to staunch Wall Street's insatiable appetite for risky derivative bets or to stop the excesses. More control is needed," said Levin in his opening statement.

At the end of the hearing Levin said the testimony painted "a very disturbing picture, not just at JP Morgan in particular, but about how derivatives are valued."

The committee heard that Dimon temporarily withheld key data from the regulator, the Office of the Comptroller of the Currency (OCC), as the bank's trading problems grew. Executives said the bank was concerned that information had been leaked in the past. "I'm not sure that there are many organisations or institutions that could get away with such a thing," said McCain.

Douglas Braunstein, JP Morgan's former chief financial officer, said: "Senator, the report in question … I am not certain, I am not aware, if it was a report required to be sent to regulators."

Drew said she believed the OCC was getting statements from the bank.

Scott Waterhouse, an OCC examiner, later flatly contradicted Drew. "We were not getting daily P&L reports on the SCP portfolio at that time." He said the regulator only started getting detailed reports again in May, after "the whole thing blew up".

Waterstone said he was at a meeting when Dimon learned from Braunstein that the reports were being sent again. "That's my decision," Dimon said, raising his voice, according to Waterstone. Thomas Curry, head of the OCC, said: "That is an unacceptable premise that the bank decides what information is provided."

Drew, in a prepared statement, said she believed her oversight had been "reasonable and diligent". She blamed flaws in JP Morgan's risk model, VaR, (Value at Risk) and "deceptive conduct by members of the London team".

According to the Senate committee, the new VaR cut the appearance of risk being taken by JP Morgan in half – even as the bank's London traders were taking ever riskier bets. As the losses mounted and press reports started to emerge, Levin suggested the bank started trying to aggressively mark down its losses.

Levin asked Braunstein whether it was appropriate for the bank to attempt to minimize its losses. Braunstein denied that this was something the bank did. "How do you possibly justify your process?" said Levin.

Levin brought up a transcript of a conversation between Drew and Javier Martin-Artajo, who supervised the London traders. "It's absolutely fine to stay conservative, but it would be helpful, if appropriate, to start getting a little bit of that mark back," she said.

Drew said that she was "challenging him" to show that the position was correct with data.

"But you used the word helpful. Right?" said Levin. "Tweaking is not a prediction or a hope, by the way. Tweaking is changing something. And I hope the guidance that you would give would be against tweaking," he said. "The suggestion was that he tweak something to make the numbers look better."

McCain quizzed the JP Morgan executives about a call with analysts last year during which Braunstein said that he was "very comfortable" with the size of the chief investment office's trades. Dimon went on to dismiss the media coverage of the losses as a "tempest in a teapot."

"Based on the benefit of hindsight, the bank and I were both misinformed and incorrect when I said I was very comfortable," said Braunstein.

The senate committee found that JP Morgan had frequently violated its own risk limits for months on end. Levin said that bank staff in London had had to manually enter information for its risk models for its massive portfolio using spreadsheets and there was very little testing of its data.

JP Morgan's acting chief risk officer Ashley Bacon said the losses had been caused by "multiple things that should have caught it didn't catch it." He said that oversight in London "failed miserably." "It would actually have been easy to catch this is many ways, and very regrettably that didn't happen," said Bacon.

But he claimed that JP Morgan did not believe it was too big to fail, and that the bank had taken many actions subsequently to make sure that something like this could not happen again.

Levin called for greater oversight of Wall Street. "The American people have already suffered one devastating economic assault rooted largely in Wall Street excess, and they cannot afford another.

"When Wall Street plays with fire, American families get burned. The task of federal regulators, and of this Congress, is to take away the matches. The whale trades demonstrate that task is far from complete," he said.